This invention relates to a method for dewatering or drying organic solid substitutes which contain large amounts of water and more particularly for removing water from carbonaceous solids, such as fossilized biomass materials including peat and coal.
Peat and certain other organic solid substrates contain substantial amounts of water. It is known that peat can contain up to 90 percent or more water, by weight, and this has been a major deterrent to its wider use as a fuel source.
It remains desirable to use peat and other biomass substrates as fuel sources by direct combustion or by high temperature conversion to a more convenient fuel, such as a fuel gas. In order to do so, however, as much of the water as possible must be removed from these materials prior to conversion since much of their potential heating value would otherwise by consumed in the evaporation of the water contained therein. For example, when peat is burned directly, much of the heat generated is used to evaporate the water originally present in the peat. As a result, the temperature produced by combustion is low and the efficiency of combustion heat transfer is greatly reduced. At a 90% water level, for example, there is essentially no available heat from peat. Available heat is the amount of heat from combustion minus the heat of vaporization of the associated water. At the 90% level, the heat required to vaporize the water is equal to or greater than the heat produced by burning the combustibles in the peat. At 80% and 70% moisture levels, only about 1000 and 1900 BTU's respectively, are produced per pound of peat. The available heat, however, increases sharply on dewatering.
In the past, water removal from carbonaceous materials, such as peat, included drying in the sun which is a slow and erratic process dependent upon variable weather conditions. Other processes, such as solvent extraction, have also been employed. For example, U.S. Pat. No. 4,339,883 discloses a process for removing water from peat or coal wherein a primary solvent is used to extract the water from the peat and a secondary solvent is used to separate the primary solvent from the water. Suitable primary solvent include alcohols, ketones, aldehydes, ethers, carboxylic acids and esters thereof, amines and amides. The patentee indicates that suitable carboxylic acids include C.sub.1 to C.sub.7 acids. The carboxylic acid esters, include ethylglycinate, tetrahydrofurfuryl acetic acid ester and carbitol acetate.
Similarly, U.S. Pat. No. 4,397,100 discloses a process for removing water from peat employing a primary solvent, preferably alcohols and a secondary solvent to remove the primary solvent from the water. Carboxylic acid esters, such as ethyl glycinate, tetrahydrofurfuryl acetic acid ester and carbitol acetate are also disclosed to be useful as primary solvents.
U.S. Pat. No. 4,014,104 discloses a process for drying particulate carbonaceous material such as coal, lignite and the like by treatment with methanol or other low heat of vaporization solvent. Similarly, U.S. Pat. No. 3,327,402 discloses a process for drying coal fines by contact with specific organic solvents, such as alkanols like methanol, ketones and similar esters.
U.S. Pat. No. 4,422,246 discloses drying fine coals by feeding the material to a pressurized zone and slurrying with a liquid water-miscible volatile organic solvent containing 1 to 4 carbon atoms. Acetone is indicated as preferred.
Notwithstanding prior art efforts, dewatering processes which are quick, reliable, energy inefficient and economical are still highly desired.